More than 50 people have been killed Sunday night when a gunman opened fire on a Las Vegas outdoor concert in a scene witnesses could only describe as nightmarish.

Perched from the 32nd floor of the Mandalay Bay hotel, a Nevada man identified as 64-year-old Stephen Paddock rained bullets on the massive crowd of about 22,000 with an automatic rifle that concertgoers say he repeatedly unloaded, paused to reload, then emptied again into unsuspecting revelers.

"The shooting would just stop... like they were reloading," a witness named Gail Davis told CBS News. "As we're running there's people laying everywhere. People were bleeding. One man was holding his girlfriend saying, 'I'm sorry,' because she was shot in the head."

While the shooter reloaded, Gail and others recall the crowd ran en masse to flee from sound of bullets that would soon start again.

When that happened, concertgoers would again hit the ground.

"We were in the front having a good time. All of the sudden we heard pop, pop, pop, pop, pop," a Canadian couple told CBS News. "Every time he stopped, we got up and made our way to the fence. And he started shooting again and we would hit the ground."

One witness guessed this happened about a dozen times.

"It kept happening and happening," said a nurse who was in attendance at the Route 91 Harvest Festival. "There were short increments where there were breaks. But it felt like, it continued for 20 minutes straight."

The nurse fled, only to turn back when her sense of duty as a health care provider kicked in.

"The guy next to us, got shot. But we went back, because I'm a nurse, and I just felt that I had to, so I went to three different scenes," she recalled. "The first one was okay, the second one was worse, and by the time I got to the third one, there were just dead bodies. Like, there's like six. And they were all young people."

Through the nightmare, the nurse said "normal citizens, doctors, cops, paramedics" who had arrived as country music fans quickly came together as practiced professionals determined to aid the injured.

"It was completely horrible, but it was absolutely amazing to see all the people come together," the nurse said.

More than 500 people were injured in the shooting, some of them critically. With the death toll already at 50 as of early Monday, the Las Vegas incident has officially replaced Orlando's Pulse nightclub massacre as the deadliest in modern American history.

Cops would later find Paddock dead of a self-inflicted gunshot wound in the hotel room, surrounded by as many as 10 firearms, Clark County Sheriff Joe Lombardo said Monday.

Authorities had been alerted to his location thanks to a smoke alarm that went off in the room, presumably triggered as a result of the shots fired.

Country megastar Jason Aldean was on stage when the shooting began. He is seen in video from the concert fleeing the stage as the shots ring out.

“Tonight has been beyond horrific,” Aldean wrote on Instagram. “I still dont know what to say but wanted to let everyone know that Me and my Crew are safe. My Thoughts and prayers go out to everyone involved tonight. It hurts my heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night. #heartbroken#stopthehate.”

The FBI and federal Bureau of Alcohol, Tobacco and Firearms were assisting local authorities with the investigation, the agencies said.

As this story continues to unfold, please check back with InsideEdition.com for updates.